A monitor receiver has the task of detecting any radioelectric emission present in a particular frequency band, referred to as a sub-band, formed by a part of the VHF band for example, and of triggering an alarm when an emission is detected. The detection of interference emissions, in particular of natural noises, is the reason for the triggering of undesirable false alarms.
The noise picked up by the receiver antenna comprises three kinds of noise:
transient noise of pulsed nature,
medium term persistent noise, of which the characteristics are liable to vary in time and as a function of the frequency band monitored,
long term persistent noise, for example of thermal origin.
A monitor receiver primarily comprises an energy detector preceded by a fixed frequency filter and a frequency changer enabling reception on a variable frequency, or preceded by a bank of filters. It is usual to couple the energy detector to a fixed threshold comparator to trigger an alarm when a signal received has an energy level exceeding the value of the threshold. This value may be preset to exceed the mean value of the long term persistent noise.
The disadvantage of a device of this kind is that the noises of the first two kinds cannot be eliminated and that it therefore has a high false alarm ratio.
The object of the present invention is to overcome this disadvantage.